This invention relates to electric power supplies and, more particularly, to power supplies having the form of a flyback switching regulator including a transformer, wherein drive control is attained by sensing an electrical parameter proportional to magnetic energy stored in the transformer.
Power supplies are constructed in numerous configurations of electrical circuits. One form of supply, useful in converting electric energy at an unregulated low voltage to electric energy at a relatively high regulated voltage suitable for charging a pulse forming network, employs a transformer in a flyback switching regulator. Circuitry at the primary winding of the transformer generates a pulse of electric voltage across the primary winding of such a polarity to reverse bias diodes connecting the transformer secondary to the pulse forming network. The current in the primary winding increases linearly with time, and is then abruptly terminated. The objective is often to terminate the primary voltage when a certain peak current has been attained. Upon termination of the primary current, the polarity of the voltage at the secondary winding causes the diodes to be forward biased connecting the secondary to the pulse forming network whereby the energy is transferred from the primary winding through the secondary winding to the load circuitry associated therewith. The pulse of energy is generally stored in a capacitor of the secondary-winding circuit for later use in applying power to an external load such as a flashlamp.
The operation of the supply continues in a repetitive fashion in which a succession of pulses of electromagnetic energy are coupled from the primary circuit of the transformer through the secondary circuit of the transformer to the load capacitor until the desired high voltage charge is obtained. The high pulse repetition frequency reduces the rate at which current is drawn by the load capacitor compared to what current would be drawn from other types of power supplies of equivalent output voltage. This permits lower peak currents to be drawn from a lower source voltage than direct charging would allow. The supply functions as a regulator by varying the pattern of pulse-width modulation to compensate for changes in input voltage and associated output voltage rise as the capacitor charges in the load circuit.
In particular, it is noted that a critical part of the regulation process relates to a determination of the status of the stored energy in the transformer. The termination of a pulse of energy to the primary is controlled according to the sensed level of energy in the transformer to prevent transformer saturation. The generation of a new pulse does not begin until a significant portion of the energy of the previous pulse has been coupled through the secondary circuit to the load circuit capacitor. Conversely, if the generation of the new pulse is delayed excessively from the previous pulse, the power output capacity may be reduced.
One form of control has employed circuitry for sensing the amplitude of current flowing in the primary circuit of the transformer. This has proven to be disadvantageous in that a current sensing resistor must be serially connected to the primary winding; also, a current sensing transformer might also be employed.
By way of example in the construction of power supplies, the following United States patents are of interest. Switching regulator power supplies are disclosed in U.S. Pat. Nos. 4,135,234; 4,180,852; 4,209,826; and 4,233,557. A saturable reactor power supply is disclosed in U.S. Pat. Nos. 4,135,234 and 3,590,362. A frequency controlled inverter power supply is disclosed in U.S. Pat. No. 3,818,314. Regulated power supplies incorporating resonance circuits are disclosed in U.S. Pat. Nos. 3,519,741; 4,030,025; 3,875,493; and 4,156,175. Current sensing in a power supply is disclosed in U.S. Pat. No. 3,663,949.
A problem exists in that both of the foregoing sensing elements present difficulties in a manufacturing operation because both are difficult to procure and to specify. For example, the current sensing resistor must be of an unusually low value of resistance, and often must have a high power rating. Low resistance reduces power dissipation but also results in millivolt levels of voltage which make the control circuit unduly sensitive to noise. In the case of the current sensing transformer, the characteristics may vary among manufacturers resulting in a loss in accuracy of control of the supply.